The present invention relates to a code error detecting circuit for high speed digitally transmitted signals, and more particularly to a circuit for detecting, in a sequence of transmitted signals into which a one-bit specific code is inserted at every m bits, the specific code and any code error in that code.
The mBlC code is well known as a code for this kind of a sequence of transmitted signals. According to the mBlC coding rule, the mBlC code is such that a one-bit specific code is inserted in specific bit position for every m bits. The speific code is a complementary code to the immediately preceding bit. An error in the mBlC code is detected as a code error in specific bit positions.
More specifically, there is generated a signal sequence in which the same code (of "1") always occurs at m-bit intervals and this is generated with exclusive ORing (EX-OR) of an mBlC-coded transmitted signal and a signal delayed by one bit from that signal. The signal sequence is supplied to a low-pass filter and a comparator to detect any error in the specific code. (See "Ultra-High Speed In-Service Error Detection Circuit" by Fujimoto et al, National Conference Record, 1985, Information and Systems, the Institute of Electronics and Communication Engineers of Japan, Toyohashi, Nov. 23-26, 1985. If there is an error in any specific code of the mBlC-coded signal sequence, a signal of "1" will, at times, be missing at m-bit intervals from the EX-OR output where it is expected. Consequently the average D.C. level will fluctuate, resulting in variation of the output level of the low-pass filter. This error is detected by detecting the transition points of the output of the low-pass filter with the comparator.
The above described code error detecting circuit of the prior art cannot, however, detect a code error if the interval between code errors is short (of error rate over 10.sup.-4) in specific codes in a sequence of transmitted signals, because the output of the low-pass filter would hardly fluctuate in this case. Moreover, since analog circuits are used in the low-pass filter and in the comparator, there are relatively many circuit elements which require adjustment.